bside the sensor is Ex approved but its GND on its body makes me suspicious.

Usually, if an EX sensor has a metallic housing, this is to be connected to earth. Inside electronics then is not connected to the metallic housing usually. Just the opposite: A test voltage of about 500V is applied between 0V of sensor circuit and metallic housing of sensor and the resulting leakage current must not exceed a certain limit.

Kai, as per your mail above a 1M resistor paralleled with 1.1....22nF capacitor is used, although it is for long running cables , but will it be beneficial to shunt the sensors floating terminal towards metallic PE, with this scheme.